Numerous types and configurations of ion sensitive electrodes are known to those skilled in the art. Such electrodes typically comprise a composition which generates an electric potential as a result of an electrochemical reaction when in contact with a solution containing the ionic species to be detected. Under certain conditions, the Nernst equation expresses the functional relationship between the magnitude of this electric potential and the ion concentration. In designing accurate electrodes, it is critical that the electrode exhibit close to the ideal Nernstian response over a large ionic concentration range. Further, the electrode potential should respond quickly to changes in ion concentration.
A wide variety of materials has been used or proposed for use in connection with the detection of ionic species. Considerable investigative effort continues to be spent on the identification and evaluation of additional electrode materials for such purposes. To produce an ion sensitive electrode having good commercial utility, it is desirable that the ion sensitive electrode demonstrate good long term electrochemical stability. More specifically, the electrical response of such material upon exposure to a particular concentration of a given ionic species should not vary significantly over long periods of time, on the order of, for example, months or years. However, an electrode material may require a significant period of time in which to equilibrate or "temper" prior to reaching a state of equilibrium from which long term deviations will be acceptably small. Therefore, it is desirable that the time to equilibrate the electrode be of as short a duration as possible.
The electrode material should have good physical resiliency and strength. In this regard, the electrode materials should not be subject to breakage upon rough handling and should not exhibit diminished electrochemical capabilities if mishandled.
Finally, in view of the trend towards the miniaturization of instrumentation, it is also desirable that the material and means of construction of the electrode allow it to be manufactured with very small physical dimensions. Such microfabrication capability increases the utility and versatility of the electrode.
The concentration of hydrogen ions in a solution, commonly referred to as pH, is an example of a chemical characteristic measured by such electrodes. The field of pH measurement, which dates back to the early 1900,'s, (see S. P. L. Sorensen, Biochem Z. Vol. 21, 131 and 201 (1909)), has been reviewed extensively, notably in the well-known books by R. G. Bates, The Determination of pH, 2nd edition (Wiley, New York, 1973) and D. J. G. Ives and G. J. Janz (eds.), Reference Electrodes (Academic Press, New York, 1961).
Certainly the most widely used device for pH measurement is the glass electrode. Because it has been studied thoroughly for several decades, its performance characteristics are well understood on the fundamental level. The glass pH electrode offers the advantages of a wide range of response, freedom from oxidation-reduction ("redox") and other interferences, and attainment of the ideal Nernstian response slope of 59 mV/pH unit. Despite these advantages, there are certain materials and design limitations (e.g., high impedance and the need for an internal aqueous phase) which preclude the straightforward microminiaturization and production-level micro-fabrication of glass pH electrodes.
Metal oxide electrodes are better suited for micro-fabrication. This class of electrochemical systems dates back to the antimony/antimony-oxide electrode (see J. M. Kolthoff and B. D. Hartong, Rec. Trav. Chim., Vol. 44, 113 (1925)), and has subsequently come to include a variety of examples.
Iridium oxide electrodes have clearly emerged as the most attractive metal oxide electrode. An iridium oxide electrode, for example, has been reported to have been used in connection with a pH-triggered Pace Maker. See, Cammilli et al., "Preliminary Experience With a pH-Triggered Pace Maker," PACE, Vol. 1, pp. 448-457 (1978). This work was performed prior to the published discovery of sputtered iridium oxide by W. C. Dautremont-Smith in 1979. See L. M. Schiavone and W. C. Dautremont-Smith, Applied Physics Letters, Vol. 35, p. 823 (1979) and Journal of the Electrochemical Society, Vol. 128, p. 1339 (1981). The preparation of anodic iridium/iridium oxide pH electrodes has also been described. See, for example, Katsube et al., "pH Sensitive Sputtered Iridium Oxide Films," Sensors and Actuators, Vol. 2, No. 4, p. 399 (1982); De Rooij et al., "The Iridium/Anodic Iridium Oxide Film (Ir/AIrOF) Electrode as a pH Sensor." in N. F. De Rooij and P. Bergveld, Monitoring of Vital Parameters During Extracorporeal Circulation, Proc. Int. Conf. Nijmegen, p. 156 (1980). However, the anodic iridium oxide electrode disclosed by De Rooij exhibits an undesirable super Nernstian electrochemical response. Moreover, it is known that anodic iridium oxides are unstable in chemically harsh environments. See Yuen, Chemical Characteristics of Anodic and Sputtered Iridium Oxide Films, Masters Thesis, University of Pennsylvania (August, 1982). Chemically oxidized iridium oxide surfaces are similarly unstable. See Dobson et al., Electrochemica Acta, Vol. 21, pp. 527-533 (1976).
In addition to iridium oxide, other oxide systems have been considered. For example, A. Fog and R. P. Buck, Sensors and Actuators, Vol. 5, p. 137 (1984) discusses the use of platinum, ruthenium, osmium, tantalum, and titanium oxides. Further, J. V. Dobson et al., Ibid. discusses the use of rhodium and zirconium oxides. Palladium oxide studies have been reported by E. Kinoshita et al., "Talanta," Vol. 33, p. 125 (1986).
Each of the above-described metal oxides except tantalum oxide and zirconium oxide is conductive and is used to measure pH in a Faradaic configuration in which a chemical change at the electrode generates a potential. Hydrogen ions are exchanged backwards and forwards between the solution and the metal oxide at a rate governed by Faraday's law to establish a thermodynamic equilibrium and a stable interface potential. Since the metal oxide is conductive, the electrical potential generated in the metal oxide by virtue of the above electrochemical reaction is constant throughout the metal oxide and can be measured by making metallic contact to the back side of the oxide.
To varying degrees, all metal/metal-oxide pH electrodes exhibit problems with redox interference. It seems that the susceptibility of a metal oxide to redox interference is to some degree correlated with the conductivity of the metal oxide such that the factors which make a metal oxide sufficiently conductive for use in a Faradaic electrode result in significant redox interference.
In contrast, insulating metal oxides, which are not capable of supplying substantial output current, tend to exhibit relatively little redox interference. However, because of their high impedance, such oxides are useful only in non-Faradaic electrode configurations which operate without substantial output current from the metal oxide. One such configuration is the ion sensitive field effect transistor (ISFET) electrode. Again the electrode is brought into contact with a solution containing the ionic species to be measured. In this configuration, the potential generated by the ion sensitive material is applied to the gate of a field effect transistor. The ISFET operates to modulate its drain to source current in response to the electric field associated with the gate's potential, without drawing significant current from the electrode. Thus, the ISFET operates on the capacitive effect of charge at the oxide-solution interface upon a transistor structure on the other side of the oxide film. Two materials that appear to be best for pH ISFET use in terms of range of response, stability, sensitivity and freedom from interference are aluminum oxide and tantalum oxide (see T. Matsuo et al., "Sensors and Actuators," Vol. 1, p. 77 ( 1981)), both of which are commonly considered to be insulators and are routinely used as passivants in electron devices and Faradaic electrochemical sensors.
Unfortunately, FET based devices are relatively complicated and more costly to fabricate than simple Faradaic pH electrodes.
It is desirable to have a pH sensitive material, capable of generating substantial output current without exhibiting redox interference. Conventional metal oxides seem incapable of achieving that end.